Velocity measurement of velocity profile of complex fluids through a microchannel and the effect of the channel size on the flow pattern

聚合物溶液通过3.71突然收缩的流动在一个微型和一个常规尺寸的通道中观察到。测试流体是分子量为6×10^6（流体A）、1.5×10^7（流体B）和1.9×10^7（流体C）的聚丙烯酰胺的0.2wt%水溶液。微通道由PDMS制成，并安装在玻璃板上。讨论了角涡的增长与剪切速率、Weissenberg数We和粘弹性马赫数M的关系。因此，对于流体B和C的流动，微通道中的涡尺寸小于常规尺寸的微通道中的涡尺寸。涡流的增长与增加的剪切速率，我们和M，但更快地通过微通道的结果相比，在常规尺寸的。而对于流体A，在微尺度通道和常规尺度通道中流动的涡增强趋势没有明显的差异。直接测量了聚合物溶液的速度分布。在深度方向上的速度分布作为一个函数的流量和壁面上的剪切应力进行了讨论。因此，对于流体A的流动，在低流速下在PDMS壁和玻璃壁之间的区域中观察到对称的轮廓，但是随着流速的增加，速度轮廓略微变得不对称：PDMS壁附近的速度大于玻璃壁附近的速度。对于流体B的流动，即使在低流速下也观察到略微不对称的轮廓，而在大流速下观察到很大程度上不对称的轮廓。PDMS壁上的这种大的表观滑移显著影响聚合物溶液通过PDMS微芯片的流动中的速度分布。

Flows of polymer solutions through a 3.71 abrupt contraction were observed in a micro- and a regular-size channels. The test fluids are 0.2 wt% aqueous solutions of polyacrylamide whose molecular weights are 6×10^6 (fluid A), 1.5×10^7 (fluid B), and 1.9×10^7 (fluid C). The microchannel was made of PDMS and it is mounted on the glass plate. The growth of the salient corner vortex is discussed as a function of the shear rate, the Weissenberg number We and the viscoelastic Mach number M. Consequently, for the flows of fluid B and C, the vortex size in the microchannel is smaller than that in the regular-size one. The vortex grows with increasing the shear rate, We and M, but more quickly through the microchannel compared with the results in the regular-size one. However, the difference of tendency of the vortex enhancement between flows in the micro-size and the regular-size channels was not observed for the fluid A. Velocity profiles of polymer solutions were directly measured. The velocity profile in the depth direction is discussed as a function of the flow rate and shear stress on the wall. Consequently, for the flows of fluid A, a symmetric profile is observed in the region between the PDMS wall and the glass wall in a low flow rate, but the velocity profile slightly becomes asymmetric with increasing the flow rate : the velocity near the PDMS wall is larger than that near the glass wall. For the flows of fluid B, a slightly asymmetric profile is observed even in the low flow rate and a largely asymmetric one in the large flow rate. This large apparent slip on the PDMS wall significantly affects the velocity profile in the flow of polymer solution through a PDMS microchip.